1. Field of the Invention
The present invention relates to timing advance for fuel injection systems of the type typically used in vehicle engines. In particular, the present invention is an improvement on the hydraulically actuated timing advance technique described in U.S. patent application Ser. No. 09/638,758 filed on Aug. 14, 2000 for xe2x80x9cTiming Advance Piston for Unit Pump or Unit Injector and Method Thereforxe2x80x9d, the disclosure of which is hereby incorporated by reference.
2. Description of the Related Art
The automotive industry is under constant pressure to reduce undesirable emissions from the internal combustion engines that power almost all vehicles currently used throughout the world. It is well known that engine emissions can be improved by adjusting the so-called xe2x80x9ctimingxe2x80x9d of the fuel injection event relative to the position of the engine piston in its engine cylinder under various engine operating conditions.
The invention is directed to a system and method by which a hydraulically actuated advance piston in a unit pump or unit injector is further modulated by a servo device. A servo device is integrated with an advance piston in the unit pump or unit injector. More particularly, an advance piston and a servo piston are nested within the cam follower of a unit pump or unit injector.
In accordance with one aspect of the present invention, a first hydraulic chamber (hereinafter the advance chamber) is defined between the advance piston and the cam follower body. A second hydraulic chamber (hereinafter the servo chamber) is defined between the servo piston and the advance piston. A relatively high, substantially constant hydraulic pressure is continuously available to the advance chamber through ports and passageways that depend on the position of the servo piston within the advance piston. The lubrication pump of an internal combustion engine may for example, generate this constant hydraulic pressure. The position of the servo piston within the advance piston is dependent upon a modulated hydraulic pressure applied to the servo chamber. Movement of the advance piston relative to the cam follower is adjusted opening and closing hydraulic ports, e.g., moving the servo piston relative to the advance piston to apply hydraulic pressure to or bleed hydraulic fluid from the advance chamber.
Preferably, the hydraulic pressure applied to the servo piston is derived from the same hydraulic source as the constant hydraulic pressure. In accordance with a particular aspect of the invention, the full hydraulic pressure produced by, e.g. an engine lubrication pump is applied to the advance piston while reduced levels of pressure from the same source are used to control application of the full hydraulic pressure to the advance piston. The full hydraulic pressure is preferably modulated in discrete increments and applied to the servo chamber to alter the position of the servo piston within the advance piston. For example, if the full hydraulic pressure available to the advance piston is 40 psi, the modulated pressure applied to the servo chamber can be any set of discrete pressures between 0 and 40 psi. A preferred embodiment of this invention will be described herein with reference to four discrete pressure levels between 0 and 40 psi, e.g., 5, 15, 25 and 35 psi. By no means is the invention limited to any particular number or values of discrete pressure levels.
In accordance with another aspect of the present invention, the fluid input port to the servo chamber is configured as a damping orifice or restricted flow opening. This damping orifice restricts the rapidity with which the servo piston can move by restricting the flow of fluid into and out of the servo chamber. The servo piston may, in the harsh environment of a cam actuated follower, have an undesirable tendency to move relative to the advance piston in response to accelerations imposed upon the cam follower by the cam, rather than the deliberate application of control pressure. A damping orifice at the entrance to the servo chamber slows movement of the servo piston relative to the advance piston, so that such relative movement takes place over several cam rotations.
One or more springs are arranged to impose a known force against the servo piston in opposition to the direction of hydraulic actuation. The spring provides a reliable means for imposing a known force on the servo piston, which is opposed by the modulated pressure delivered to the servo chamber. A differential between the servo spring force and the pressure in the servo chamber determines the position of the servo piston within the advance piston bore. By connecting the advance chamber to hydraulic pressure (advance) or alternatively to a bleed passage (retard), the servo piston position determines the volume of the advance chamber and, ultimately, the position of the advance piston relative to the cam follower.
The discrete modulation of the hydraulic pressure to the servo piston is preferably translated into discrete and predictable advance piston positions by the use of hydraulic porting and passageway configurations that open and close precisely in response to displacement of the advance piston relative to one or both of the follower body and servo piston. Use of porting with edges acting as valves achieves more precise control of multiple discrete advance positions than is available from reliance solely on hydraulic pressure modulation from, e.g., a proportional solenoid valve.
The net force acting on the advance piston is proportional to the difference between the pressure in the advance chamber and the pressure in the servo chamber (which is proportional to the force exerted by the servo spring on the servo piston). As the advance chamber decreases or increases in volume, the advance piston is displaced toward or away from the pumping plunger, thereby affecting the return or rest position of the plunger and thus the timing of an injection event.
The integration of the advance piston, servo piston, servo spring, and associated porting and passageways into the follower body to form a compact cam follower assembly, represents another aspect of the invention. This integration is facilitated by incorporation of an advance piston cap resting on a shoulder formed near the upper end of the advance piston. The servo spring seat is in the form of a generally cylindrical body coaxially received within the cap and the servo piston. The cap and the advance piston are shaped to generously accommodate a transversely oriented holding pin anchored in the follower body and closely penetrating the servo spring seat. The spring seat is thereby fixed in relation to the follower body, but the advance piston and associated cap can move relative to the follower body and pin. The integration is further implemented by hydraulic ports and passageways penetrating the cylindrical wall of the follower body, selectively alignable with ports and passageways through the cylindrical wall of the advance piston, which in turn are selectively alignable with annular fluid transfer channels on the outer surface of the servo piston.
An object of the present invention is to provide a new and improved servo controlled advance piston for a unit pump or injector that provides a greater degree of control over the timing of an injection event.
Another object of the present invention is to provide a new and improved servo controlled advance piston for a unit pump or injector that improves the performance of an internal combustion engine equipped with the servo controlled advance piston for a unit pump or injector.
A further object of the present invention is to provide a new and improved servo controlled advance piston for a unit pump or injector that reduces undesirable exhaust emissions from an internal combustion engine equipped with the servo controlled advance piston for a unit pump or injector.
A yet further object of the present invention is to provide a new and improved servo controlled advance piston for a unit pump or injector that integrates control of injection duration with control of injection timing.
These and other objects, features, and advantages of the invention will become readily apparent to those skilled in the art upon reading the description of the preferred embodiments, in conjunction with the accompanying drawings.